If the configuration option is not selected (see point no. 2), calling FB55 does not succeed. Provided for such a case in the manual is a specific error message (STATUS) that is also supplied by the S7-300 series CPs.
STATUS = 0x8B10
"There is no setting for configuring via the user program."
Error message of the S7-400 CP:
With 400 series CPs the STATUS = 0x80A4 is returned. This status value is not documented for FB55 and comes from a lower-level SFC. It means:
"The K bus connection between CPU and CP is not set up".
The status value in this case is equivalent to status value 0x8B10 for S7-300 described in the manual.
Reason for the differences between S7-300 and S7-400:
The different status values are due to the system-specific realization of data transmission between CPU and CP in S7-300 and S7-400 respectively.